1. Technical Field
The present disclosure generally relates to the conversion of analog signals into digital signals and, more specifically, to such a conversion applied to signals issued by image sensors.
2. Description of the Related Art
Analog-to-digital converters are used in image sensors to convert an analog signal representative of the light intensity sensed by a photodiode into a digital signal to be stored and processed in digital circuits. The accuracy of the information stored in digital form depends, not only on the accuracy of the analog circuits, but also on the number of bits into which this analog signal is converted. However, the larger the number of bits of the converter, the more space said converter takes up or the longer the conversion time or both.
Two types of noise are likely to influence the signal-to-noise ratio of the converter. For a low-level signal, a so-called read noise, linked to the acquisition chain and independent from the signal level, predominates. The level of this noise conditions the analog equivalent of the least significant bit. Noise linked to the signal adds thereto. This noise, called the shot noise, is proportional to the square root of the number of electrons received by the sensor.
In an image sensor, a converter is generally assigned to each pixel column of the sensor and conversions are performed simultaneously for the pixels of a same line. Accordingly, when the number of conversion bits is increased, the resulting size increase is to be multiplied by the number of columns and the processing time increase is to be multiplied by the number of lines.
On the analog signal side, once the read noise has become smaller than the shot noise, the signal-to-noise ratio is equal to N/√N, where N represents the number of electrons received by the sensor.
It has already been provided to take into account the shot noise in an image sensor by varying the slope of the ramp of a ramp converter. This amounts to settling, for high-level signals, for a coarser resolution than with a low-level signal. Actually, the ramp slope increases along time over a measurement period. Such a solution is described in the article “A Low-Power Column-Parallel 12-bit ADC for CMOS Imagers” by M. F. Snoeij et al. (Delft University of Technology) published in June 2005.
For a given bulk, a variable-ramp solution does not increase the number of bits provided by the converter, but it provides a benefit in time by decreasing the number of quantization levels. Further, the response time of the converter comparators depends on the ramp slope. The variable ramp is thus likely to generate a quantization error.
It would be desirable to be able to increase the resolution of an analog-to-digital converter without for all this increasing its complexity or its processing time.
As an example, it is more and more desired to pass from 10 to 12 bits for digital signals provided by image sensors.